As a material for an arc tube of a high-pressure discharge lamp, translucent alumina has been conventionally used in place of quartz because translucent alumina has high heat resistance temperature of 1200° C. or higher and is produced relatively inexpensively.
However, a high-pressure discharge lamp using an arc tube made of an alumina sintered body has a short life of approximately 9000 hours. This is mainly due to the low thermal shock resistance of the alumina sintered body. In addition, in the case of a metal halide lamp, since alumina has low corrosion resistance to a metal halide gas of alumina, the life of the lamp is further reduced.
Consequently, in recent years, there have been developed arc tubes made of an aluminum nitride sintered body having a high translucency. Since the aluminum nitride sintered body is more excellent in thermal shock resistance and corrosion resistance than the alumina sintered body, the life of the lamp may be increased.
However, although excellent in corrosion resistance, the aluminum nitride sintered body has a problem of poor wettability with many molten metals. For this reason, it is difficult to strongly bond an arc tube made of an aluminum nitride sintered body and an electrode so that the gas tightness in the arc tube is kept high and the gas trapped in the arc tube does not leak.
In Patent Document 1, in order to increase gas tightness in an arc tube, an attempt has been made to bond an arc tube made of an aluminum nitride sintered body and an electrode through a tungsten layer and a layer of a brazing filler metal containing Nd2O3, Y2O3 and aluminum nitride.
However, this method requires two steps in which firstly a tungsten paste is applied on the arc tube and then the brazing filler metal is applied on the paste.
In addition, in Patent Documents 1 and 2, an attempt has also been made to directly bond an arc tube of an aluminum nitride sintered body and an electrode of tungsten or molybdenum by integrally sintering them without using any paste.
However, the arc tubes obtained by the above direct bonding have limited gas tightness.
Further, Non-Patent Document 1 describes that the transmittance of an aluminum nitride sintered body is reduced when impurities such as Zr, Co, Nb, Ta, W, Mn, Hf, Cr, Fe and Ti are incorporated in the sintered body.    [Patent Document 1] Japanese Patent Application Laid-Open Publication No. H02-189853    [Patent Document 2] Japanese Patent Application Laid-Open Publication No. H06-290750    [Non-Patent Document 1] J. Am. Ceram. Soc., 77 (8) 1991-2000 (1994)
An object of the present invention is to solve the problems involved in the conventional technology as mentioned above and is to provide a cement for bonding an arc tube body made of an aluminum nitride sintered body and an electrode support made of molybdenum, wherein the arc tube produced has high gas tightness without impairing the excellent translucency of the aluminum nitride sintered body.
Furthermore, another object of the present invention is to provide an arc tube having excellent translucency and corrosion resistance in which the arc tube body and the electrode support are integrated using the above-mentioned cement, and a high-pressure discharge lamp having a long life in which the arc tube is used.